Sure. For example, libgnomeui-devel already depends on GConf2-devel,
glib2-devel, and gnome-vfs2-devel, so you don't have to list those explicitely.
All the BR I have listed are already required by other BRs, so you can remove them.

OK
I yet leave gtk2-devel and GConf2-devel, because they requires '>= 2.4.0', but
implicit dependencies give '>= 2.3.0' only.
New SRPM and SPEC at the same location (comment #2) (No release increment due to
nobody still download it... :-))

Sooner or later you will run into the trap of not updating the versioned
Buildrequires, and then you will be out-of-sync. Notice that even FC2 comes
with a version of gtk2-devel, which is new enough, so all the distributions
your Fedora Extras package targets, are sufficient.
Versioned [build] dependencies make much more sense, when the package from
original version of a distribution is insufficient, but an Update is
available and required.

For comment #5 :
It is possible that someone will rebuild this package under some "old enough"
distribution (fc1, rhl7.3 etc.), or even under some another Fedora-based
distribution. Therefore I think this versioning is a little bit useful.
I hope that it is not required to update these dependences in the future (at
least, visible future), because it is ">=", not "==" ...

> No release increment due to nobody still download it...
Please do increment. It helps when saying "Release x approved", and it also
helps the reviewer.
> I hope that it is not required to update these dependences in the future
If the upstream project depends on newer libs, you'll have to reflect this in
your RPM. But if you're willing to follow that, that's fine. You're the
maintainer in the end.
Last problem in the spec file: since it installs a scrollkeeper file, you have
to add the required bits in the scriptlets. Please see
http://fedoraproject.org/wiki/ScriptletSnippets, scrollkeeper section.